The invention is directed to a non-destructive wafer probing system for characterizing monolithic microwave integrated circuits (MMICs). FETs and other devices, and more particularly to such a system for characterizing these devices as a function of signal frequency up to at least 100 GHz.
Gallium arsenide (GaAs) microwave and millimeter-wave devices and monolithic integrated circuits are currently being developed for applications such as satellite communications, radar, and phased-array systems. The conventional frequency domain approach for testing these discrete devices and MIMIC chips usually constitutes one of the major costs of a development program. This is especially true for devices and circuits operating in the millimeter-wave regime. Therefore, a low-cost testing technique which allows on wafer characterization of MMICs before dicing the wafer into individual chips is highly desirable.
In addition to the cost factor, the evaluation of these devices becomes more difficult as applications extend to the higher millimeter wave frequencies. Fairly accurate characterization of active devices and monolithic circuits can be achieved by using a waveguide measurement system that offers small attenuation, and with careful design may achieve low mismatch loss. However, this approach is inherently limited by the waveguide bandwidth, resulting in the need for multiple calibrations for different waveguides when measurements are made over a wide frequency range. Furthermore, high-performance waveguide-to-microstrip transitions for each waveguide band, and careful assembly of the interface between these transitions and the MMIC test block, are required. Such an evaluation process is quite time consuming. In addition, oscillations may occur during the characterization of active devices that are not terminated with matching circuits. This problem occurs because of the purely reactive termination presented by the waveguide below its fundamental mode cutoff frequency.
Current commercially available on-chip characterization systems for MMICs have provided useful performance data in the lower microwave range, and attempts are being made to extend their frequency of operation. However, several fundamental limitations still exist. Because these systems use special coplanar waveguide (CPW) probes. e.g.. as disclosed by E. W. Strid. "26-GHz Wafer Probing for MMIC Development and Manufacture," Microwave Journal, pp. 71-82, August 1986, it is difficult to achieve a low-loss, impedance-matched probe at millimeter-wave frequencies. The operating life of such a mechanical direct-contact probe is usually quite limited, and a customized probe card is required for each set of microwave circuits on the wafer. The circuits also require CPW patterns to be incorporated at various test locations on the wafer.
Recently, optical techniques have been used in the characterization of microwave devices and circuits. Frequency domain measurements have been performed using electro-optic probing of a microstrip line. e.g. as disclosed by B. Kolner et al. "Electro-optic Sampling in GaAS Integrated Circuits." IEEE J. Quantum Electron., Vol. QE-22, pp. 79-93, January 1986 and K. J. Weingarten et al, "Picosecond Optical Sampling of GaAs Integrated Circuit." IEEE J. Quantum Electron., Vol. QE-24. pp. 198-220, February 1988. In this work, the microwave signal was launched onto the circuit using CPW contacting probes. J. A. Valdmanis et al. "Subpicosecond Electrical sampling and Applications." Picosecond Optoelectronic Devices, Academic Press, pp. 209-270, 1984, have demonstrated that, by using an electro-optic probe containing polar material such as lithium tantalate, substrates which do not exhibit the electro-optic effect can still be probed. Some results for a GaAS field-effect transistor (FET) mounted on a silicon-on-sapphire (SOS) test circuit have been presented by D. E. Cooper et al, "Picosecond Optoelectronic Measurement of the High-Frequency Scattering Parameter of a GaAS FET." IEEE J. Quantum Electron., Vol. QE-22, pp. 94-100, January 1986.